Inorganic fullerene-like nanoparticles and nanotubes are attractive due to their unique crystallographic morphology and their interesting physical properties. Molybdenum disulfide belongs to a class of solid lubricants useful in vacuum, space and other applications where liquids are impractical to use.
It is known to obtain carbon fullerenes and nanotubes by a synthetic process consisting of condensation of molecular clusters from the vapor phase. This is disclosed for example in the following publication:    (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press, INC, 1996; pp 1-6, 110-116;    (2) Flagen, R. C.; Lunden, M. M. Materials Sc. Eng. A 1995, 204, 113;    (3) Deppert, K.; Nielsch, K.; Magnusson, M. H.; Kruis, F. E.; Fissan, H. Nanostructured Materials 1998, 10, 565; and    (4) Kruis, F. E.; Fissan, H.; Peled, A. J. Aerosol Sci. 1998, 29, 511.
According to this technique, a hot vapor is quenched and entrained by a flowing inert gas. Nanoclusters are obtained by an adiabatic expansion of the vapor leading to a cooling down of the clusters-inert gas vapor and its condensation. Fullerene-related nanoparticles are derived from materials with a layered structure.
There are three main types of fullerene-related particles: fullerenes (C60,C70,etc.); nested-fullerene nanoparticles (onions), and nanotubes. Analogous fullerene-like nanoparticles can be obtained from a number of inorganic materials with layered structure, which are known as inorganic fullerene-like materials-IF.
Fullerenes can be produced from carbon-rich vapor or plasma, which can be obtained using laser ablation, arc discharge or resistive heating of graphite targets, as disclosed in the above publication (4), as well as in the following publications:    (5) Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smally, R. E. Nature 1985, 318, 162;    (6) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, R. Nature 1990, 347, 354.
Techniques for obtaining fullerene-like nanoparticles of MoS2 from MoS2 powder subjecting the powder to e-beam irradiation and laser ablation are disclosed in the following publications:    (7) Jose-Yacaman, M.; Lorez, H.; Santiago, P.; Galvan, D. H.; Garzon, I. L.; Reyes, A. Appl.Phys. Lett. 1996, 69, 8, 1065;    (8) Parilla, P. A.; Dillon, A. C.; Jones, K. M.; Riker, G.; Schulz, D. L.; Ginley, D. S.; Heben, M. J. Nanure 1999, 397, 114.
According to another known techniques, short electrical pulses from the tip of a scanning tunneling microscope over a film consisting of amorphous MoS3 nanoparticles, can be used to cause the formation of nanoparticles with a closed MoS2 shell (IF), a few molecular layers thick, and amorphous MoS3 core. This is disclosed for example in the following publication:    (9) Homyonfer, M.; Mastai, Y.; Hershfinkel, M.; Volterra, V.; Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 33, 7804.
A technique of obtaining closed cages and nanotubes of NiCl2 has been recently developed and disclosed in the publication:    (10) Rosenfeld-Hacohen, Y., Grinbaum, E., Sloan, J., Hutchison, J. L. & Tenne, R., Nature 1998, 395, 336.
According to the above technique, heating of NiCl2 to 960° C. in a reducing atmosphere is utilized.
As recently reported by Chhowalla and Amaratunga in Nature 407, 164 (2000), clusters of hollow fullerene-like MoS2 “onions” can be produced in the form of thin films.
IF nanoparticles, including nanotubes, may also be obtained by chemical methods. The first synthesis of IF-MS2 (M=Mo,W) is based on the sulfidization of the respective amorphous MO3 thin films in a reducing atmosphere at elevated temperatures (˜850° C.). This technique has been developed by the inventor of the present application, and are disclosed in the following publications:    (11) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444;    (12) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature 1993, 365, 113;    (13) Hershfinkel, M.; Gheber, L. A.; Volterra, V.; Hutchison, J. L.; Margulis, L.; Tenne, R. J. Am. Chem. Soc. 1994, 116, 1914.
As further found by the inventors of the present application, the synthesis of IF-MoS2 including MoS2 nanotubes can be carried out by using molybdenum oxide powder instead of a thin film precursor. This is disclosed in the following publication:    (14) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222.
However, the above synthesis resulted in miniscule amounts of the nanoparticles and a limited size control.
More recently, macroscopic quantities of IF-WS2 and WS2 nanotubes were obtained from a powder of tungsten oxide nanoparticles. These techniques are disclosed in the following publications:    (15) Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya, V.; Margulis, L.; Cohen, H.; Hodes, G.; Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 5362.    (16) Rothschild, A.; Frey, G. L.; Homyonfer, M.; Tenne, R.; Rappaport, M. Mat. Res. Innovat. 1999, 3, 145.
The mechanism for the synthesis of IF-MS2 (M=Mo, W) from the oxide powder that adequately describes the growth of IF-WS2 from WO3 nanoparticles, as found by the inventors of the present application, was more specifically disclosed in the article:    (17) Feldman, Y.; Lyakhovitskaya, V.; Tenne, R. J. Am. Chem. Soc. 1998, 120, 4176.
According to this mechanism, within the first few seconds of a chemical reaction, an encapsulate consisting of a skin of monomolecular MS2 layer or two with a suboxide core, is formed. It was shown that the kinetics of the sulfidization/reduction processes on the surface of the oxide nanoparticles vary strongly with temperature. Only in the temperature range 700-850° C. do the kinetics of the reaction allow sufficiently rapid generation of an absolutely closed spherical sulfide monolayer. This sulfide monolayer averts the fusion of the oxide nanoparticles into micron-size particles and promotes the growth of concentric spherical layers, characterizing fullerene-like structures. Later on, the suboxide core is progressively converted into IF sulfide layers by a slow diffusion-controlled reaction. Consequently, the size and shape of the IF particles are determined by the size and shape of the incipient oxide nanoparticles. It is important to note that the size of the oxide nanoparticles must not exceed 300 nm, otherwise 2H platelets of the respective sulfide are obtained.
MoS2 and WS2 nanotubes were also obtained by chemical vapor transport of the MS2 powder with iodine as a transporting agent, as disclosed in the following articles:    (18) Remskar. M.; Skraba, Z.; Cleton, F.; Sanjines, R.; Levy, F. Appl. Phys. Lett. 1996, 69, 351.    (19) Remskar, M.; Skraba, Z.; Regula, M.; Ballif, C.; Sanjines, R.; Levy, F. Adv. Mater. 1998, 10, 246.
In an another approach, aquaeous solution of ammounium thiomolybdate was soaked into a porous alumina template. This is disclosed in the article:    (20) Zelenski, C. M.; Dorhout, P. K. J. Am. Chem. Soc. 1998, 120, 734.
Subsequent annealing led to the formation of MoS2 nanotubes, which were isolated by dissolving the alumina matrix with KOH solution.
More recently, sonochemical methods have been used for the synthesis of IF structures from various inorganic compounds, including Tl2O. This is known from the following publication:    (21) (a) Mastai, Y.; Homyonfer, M.; Gedanken, A.; Hodes, G. Adv. Mater. 1999, 11, 1010; (b) Avivi, S.; Mastai, Y.; Hodes, G.; Gedanken, A. J. Am. Chem. Soc. 1999, 121, 4196; (c) Avivi, S., Mastai, Y., Gedanken, A. J. Am. Chem. Soc., 2000, 122, p. 4331.